PHYSIOLOGY 171 



tain cells of the intestinal lining, causing them to secrete into the 

 blood a substance of hormone nature, known as secretin. Upon 

 reaching the pancreas this secretin stimulates it to secrete the diges- 

 tive fluid, pancreatic juice, into the small intestine by way of the 

 pancreatic ducts. There is some evidence that secretin also stimu- 

 lates secretion in the liver. 



Pancreatic juice is a clear, watery, alkaline solution containing 

 inorganic salts (carbonates, etc.) and three enzymes; the protease, 

 trypsin, the diastase amylopsin, and the lipase, steapsin. These act 

 respectively on proteins and peptones, starches and sugars, and fats. 

 This protease is in the form of trypsinogen until it reaches the intes- 

 tine and is activated by an intestinal enzyme, enterokinase. Trypsin 

 completes the work begun by the pepsin in that it converts proteoses 

 and peptones into amino acids, but it also digests complex proteins 

 which have escaped the action of pepsin. It acts more rapidly and 

 efficiently than does pepsin. There are nineteen amino acids that 

 are regarded as building stones of the protein molecule. In a com- 

 plex protein like casein, as many as sixteen of these amino acids will 

 be found. The tissues of the animal body must not only have avail- 

 able a wide range of amino acids but must also select in the proper 

 proportion the ones needed to reconstruct their specific protein con- 

 stituency. 



Amylopsin is the pancreatic diastase, and it is able to bring about 

 hydrolysis of carbohydrates in the alkaline niedium of the intestine 

 without activation. It produces dextrin and maltose (malt sugar). 

 The pancreatic lipase, steapsin, brings about the splitting of fats 

 into glycerin (glycerol) and one or more fatty acids, such as stearic 

 acid, oleic acid, butyric acid, etc. The alkaline salts, which are in- 

 troduced by the bile, combine with these fatty acids to form soaps 

 which help in emulsifying the remaining fats, thus making them more 

 readily split. 



Intestinal secretions or succus entericus which are produced by 

 glands in the mucous membrane of the small intestine include five 

 enzymes. Enterokinase, which activates trypsinogen to form trypsin, 

 has been mentioned already. Erepsin, the intestinal protease, supple- 

 ments the activity of trypsin by converting proteoses and peptones 

 into amino acids. Maltase converts maltose and dextrin into dextrose 

 (glucose). Invertase changes sucrose (cane sugar) into dextrose and 

 levulose. Lactase converts milk sugar (lactose) into galactose and 

 dextrose, both simple sugars. 



